CN114303209A

CN114303209A - Current lead for superconducting magnet

Info

Publication number: CN114303209A
Application number: CN202080060178.9A
Authority: CN
Inventors: 西蒙·乔利; 亚当·保罗·约翰斯通
Original assignee: Siemens Healthineers Ltd
Current assignee: Siemens Healthineers Ltd
Priority date: 2019-09-04
Filing date: 2020-07-16
Publication date: 2022-04-08
Also published as: GB2586821B; GB2586821A; GB201912698D0; WO2021043486A1; US20240096535A1

Abstract

A current lead arrangement for providing current to a superconducting magnet coil (10) includes a current lead (14) and a cryocooler (16). The current lead (14) includes a first portion of Low Temperature Superconductor (LTS) wire (14a) bonded to a second portion of High Temperature Superconductor (HTS) material (14b) and, in turn, to a third portion of resistive material (14 c). The cryocooler (16) comprises a first cooling stage (18) and a second cooling stage (20). The lower end of the third portion (14c) and the upper end of the second portion (14b) are thermally coupled to the first cooling stage (18), and the lower end of the first portion (14a) is thermally and electrically connected to the superconducting magnet coil (10).

Description

Current lead for superconducting magnet

Technical Field

The present invention relates to superconducting magnets and in particular to current leads for carrying current between a superconducting magnet and a source of current which is itself at room temperature.

Background

Currently known superconducting materials are generally classified into "Low Temperature Superconductors" (LTS) having a superconducting transition Temperature below about 20K and "High Temperature Superconductors" (HTS) having a superconducting transition Temperature above about 20K.

Disclosure of Invention

The invention is particularly concerned with superconducting magnets comprising coils of superconducting wire with LTS cores, but the invention may be applied to superconducting magnets comprising coils of superconducting wire with HTS cores. Typically, both superconducting wires with LTS cores and superconducting wires with HTS cores have copper or aluminum stabilizers and may benefit from the application of the present invention.

Fig. 1 schematically shows a conventional arrangement for cooling superconducting magnet coils and providing galvanic connections to and from the magnet coils. As shown in fig. 1, magnet coils 10, which consist of coils of superconducting wire, which themselves comprise one or more superconducting filaments in the form of a non-superconducting metal matrix, such as copper or aluminum, are connected to electrical conductors 12 by current leads 14. The current lead 14 includes a first portion 14a lts superconductor, a second portion 14b hts superconductor, and a third portion 14c non-superconducting conductor such as copper, brass, or aluminum. Typically, two such current lead arrangements will be provided, one for providing current to the magnet coil 10 and one for providing a return path for the current to a current supply not shown, but for ease of representation only one current lead arrangement is shown. LTS portion 14a typically includes a superconducting wire having a stable matrix of copper or aluminum in which a plurality of superconducting cores are provided.

The superconducting magnet coils 10 are cooled by a refrigerator 16. The refrigerator 16 has a first cooling stage 18 which cools to a first cryogenic temperature, typically in the range of 25K to 80K, and also has a second cooling stage 20 which cools to a second cryogenic temperature, typically about 4K. The first cooling stage typically cools the thermal shields within the cryostat, and the second cooling stage typically cools the superconducting magnet coils 10 to operating temperature.

Thus, in operation, the third portion 14c of the current lead 14 extends between a room temperature of approximately 300K to a first cryogenic temperature of 25K to 80K. The rate of heat transfer through the third portion 14c of the current lead 14 will be determined by the material, length and cross-sectional area of the third portion and the temperature difference between the two ends of the third portion.

In operation, the second portion 14b of the current lead 14 extends between a first cryogenic temperature of 25K to 80K and a second cryogenic temperature of about 4K. The rate of heat transfer through the second portion 14b of the current lead 14 will be determined by the material, length and cross-sectional area of the second portion and the temperature difference between the two ends of the second portion.

In operation, the first portion 14a of the current lead 14 extends between the second cooling stage 20, which is at the second cryogenic temperature of about 4K, and the magnet coil 10, which is also at about 4K. The rate of heat transfer through the first portion 14a of the current lead 14 will be determined by the material, length and cross-sectional area of the third portion, but this heat transfer rate should be minimal because there should be a very small temperature difference between the two ends of the third portion.

A solid or fluid thermal link 22 of an electrically conductive material, such as high purity aluminum or high purity copper or a thermosiphon or a combination thereof, thermally links the second cooling stage 20 to the magnet coils 10. Preferably, and as shown, the thermal switch 24 may be placed in the thermal path between the second cooling stage 20 and the magnet coil 10. Examples of suitable thermal switches 24 include thermosiphons, heat pipes, air gaps, solid (e.g., reluctance) or mechanical switches.

HTS current leads, such as second portion 14b of current lead 14, are required for low-cryogen and "dry" (no liquid cryogen path) superconducting magnet systems so that current can be carried into and out of the superconducting magnet with minimal dissipation. As described above, and generally, the top of the HTS portion 14b is thermally anchored to the first cooling stage 18 of the cryocooler 16 by a high thermal conductivity link, while the lower end of the HTS portion is thermally anchored to the second cooling stage 20 of the cryocooler 16 by a high thermal conductivity link. The lower end of the HTS part is also thermally linked to the magnet coil 10 by a high-conductivity part 14a, the high-conductivity part 14a typically being an LTS wire.

Heat flowing from electrical conductor 12 through third portion 14c of current lead 14 at room temperature is intercepted and removed by first cooling stage 18 of refrigerator 16.

Heat flowing down the second portion 14b of the current lead 14 from the first cooling stage 18 of the refrigerator at 25K to 80K is intercepted and removed by the second cooling stage 20 of the refrigerator 16.

In the event of a failure of the chiller 16, the second stage 20 of the chiller will warm up rapidly because the heat capacity of the material is low at the operating temperature of the second stage. In the event that the refrigerator 16 is not operating for any reason, heat from the room temperature end will be conducted through the material of the refrigerator to the second cooling stage 20 of the refrigerator. Although the thermal switch 24 preferably blocks heat conduction from the second cooling stage 20 through the heat link 22 to the magnet coils 10 in the event of a chiller failure, heat will still be conducted from the second cooling stage 20 to the magnet coils 10 through the first portion 14a of the current lead 14, i.e., the LTS portion. The first cooling stage 18 of the refrigerator will also be significantly warmed and heat from this first cooling stage 18 will be conducted down through the HTS portion 14b of the current lead 14 to the LTS portion, i.e. the first portion 14a, of the current lead 14 and thence to the magnet coil 10. In general, an LTS wire such as may be used for the LTS section 14a of the current lead 14 comprises a plurality of superconductor filaments of small cross-section and a stable matrix material of significantly larger cross-section, such as aluminum or copper, and has a considerable thermal conductivity. These heat flows to the magnet coils 10 will cause a quench which results in a long shutdown of the superconducting magnet, since the magnet coils 10 must be re-cooled before current can be introduced, and heat transfer at such low temperatures is often inefficient.

Conventional methods of avoiding such magnet quench in the event of a chiller failure include:

providing a backup power supply and cooling water can be used to overcome the effects of service failures and keep the refrigerator running. However, this arrangement is expensive because of the large, uninterrupted power supply or generator required, as well as the spare water coolers or air cooled compressors.

Heat capacity can be added to the magnet coils 10 to limit the temperature rise in response to a given heat inflow. Example arrangements may employ a liquid cryogen such as helium, a solid with a high heat capacity such as a rare earth metal, or a more general material of greater mass such as copper. However, such materials can be very expensive and may require a large mass to significantly change the heat capacity of the magnet coil 10.

For small magnet systems one can simply accept that the superconducting magnet coils 10 will quench within a few minutes after the refrigerator stops running. For a small magnet which cools down again within a few hours, and

or in cases where user uptime is not important, this may be acceptable. For a whole-body MRI system, it is expected that uptime will be 99% or more, and where the recooling time may be 0.5 to 2.0 days or more, this pose would be unacceptable.

Accordingly, the present invention provides an improved current lead for a superconducting magnet that reduces the amount of heat influx into the superconducting magnet coils in the event of a refrigerator failure.

The present invention provides an apparatus as defined in the appended claims.

Drawings

The above and further objects, features and advantages of the present invention will become more apparent from the following description of specific embodiments thereof, given by way of non-limiting example in conjunction with the accompanying drawings, wherein:

FIG. 1 schematically illustrates a conventional arrangement for providing current to a superconducting coil;

fig. 2 schematically shows an arrangement for supplying current to a superconducting coil according to a first embodiment of the present invention;

fig. 3 schematically shows an arrangement for supplying current to a superconducting coil according to a second embodiment of the present invention; and

fig. 4 schematically shows an arrangement for supplying current to a superconducting coil according to a third embodiment of the present invention.

Detailed Description

According to the present invention, an alternative arrangement for providing current to the magnet coil 10 is provided, and accordingly a return path is provided for the current from the magnet coil 10.

Fig. 2 to 3 illustrate an alternative embodiment of the present invention. The present invention provides such an arrangement: wherein no low thermal resistance path is provided between the second cooling stage 20 of the refrigerator and the superconducting magnet coil 10. Features of fig. 2 to 3 which are identical to those of the arrangement of fig. 1 bear corresponding reference numerals.

In the embodiments of fig. 2 to 3, and for the arrangement of fig. 1, two similar current leads will typically be provided: one to provide current to the magnet coil 10 and one to provide a return path for current from the magnet coil 10. In each case, however, only one current lead is shown for ease of illustration.

In the embodiment of fig. 2, there is no thermal link between the second cooling stage 20 and the current lead 14. Current lead 14 still includes a first LTS section 14a, a second HTS section 14b, and a third non-superconducting section 14 c. In the conventional arrangement of fig. 1, the lower end of the second HTS portion 14b and the upper end of the first LTS portion 14a are thermally coupled to a second cooling stage 20. In the embodiment of fig. 2, there is no thermal link between the current lead 14 and the second cooling stage 20 of the refrigerator. As schematically illustrated, this may be achieved simply by mechanically separating the current lead 14 from the second cooling stage 20. Alternatively, a solid thermal insulation material 26 may be provided between the current lead 14 and the second cooling stage 20 to provide mechanical support to the current lead 14 without allowing heat to be transferred from the second cooling stage 20 to the current lead. Suitable materials for use may be selected from the following: plastics such as nylon, PTFE, polyester; composite materials such as GRP, carbon fiber reinforced plastic, G10, Durastone, gossypol aldehyde; small cross section stainless steel.

In an exemplary embodiment, the first LTS section 14a includes NbTi superconducting filaments that remain superconducting at temperatures up to about 8K under typical current and background fields. In the absence of a thermal link to the second cooling stage 20, it may be necessary to increase the thermal conduction of the first LTS portion 14a of the current lead 14, as the LTS portion 14a must be cooled, in use, to superconducting temperatures, such as 8K or less, by conduction through its own length and then through the main magnet thermal bus, i.e. the solid thermal link 22. This may be accomplished by adding more highly thermally conductive material, such as aluminum or copper, to the first LTS portion 14a of the current lead 14. For example, a length of copper wire of a desired cross-section may be soldered in parallel to the copper sheathed superconducting wire used for the first LTS section 14 a. Such copper wire may be in the form of an extra length of sheathed LTS wire. Alternatively, a desired copper or aluminum jacket size superconducting wire may be used for the first LTS portion 14 a.

In operation of the magnet coil 10, no current flows in the current lead 14. The current leads 14 reach a thermal equilibrium determined by the temperature of the first cooling stage 18, the temperature of the second cooling stage 20 and the thermal resistance of the main magnet thermal bus, i.e. the solid heat link 22 and the magnet coils 10.

In the event of a failure of the refrigerator 16, heat conducted from room temperature through the refrigerator 16 to the second cooling stage 20 can only flow to the magnet coils 10 through the main magnet thermal bus, i.e. the solid heat coupling 22. If, as is preferred, the thermal switch 24 is provided in the solid state heat link 22, the thermal switch 24 can be opened to prevent heat transfer from the second cooling stage 22 to the magnet coils 10 in the event of a chiller failure. Thus, there will be no thermal path from the second cooling stage 20 to the magnet coils 10 or the first LTS portion 14a of the current lead 14. Heat transfer from the first stage 18 does not significantly heat the magnet coil 10 or the LTS first portion 14a of the current lead 14 because the HTS material of the second HTS portion 14b of the current lead 14 has a high thermal resistance.

In the embodiment of fig. 3, second HTS portion 14b of current lead 14 extends beyond second cooling stage 20. Corresponding to the conventional arrangement of fig. 1, the thermal bond between second cooling stage 20 and current lead 14 is maintained, but in the embodiment of fig. 3, the thermal bond between second cooling stage 20 and current lead 14 is formed partially along second HTS portion 14 b. The lower end of second HTS part 14b and the upper end of first part 14a are thermally insulated from second cooling stage 20 by a significant thermal resistance due to the high thermal resistance of the HTS materials.

In operation of the superconducting magnet, no current flows through current leads 14, thermal switch 24 is closed, if thermal switch 24 is present, and magnet coils 10 and first LTS portion 14a of current leads 14 are cooled by second cooling stage 20 to the LTS superconducting temperature by conduction of heat through the main magnet thermal bus, i.e., solid thermal link 22.

In the event of a chiller failure, the thermal switch 24 may be opened, if the thermal switch 24 is present. No heat will then flow from the second cooling stage 20 to the magnet coils 10 through the main magnet thermal bus, i.e. the solid heat link 22. Although heat will be transferred through the structure of the refrigerator 16 to the second cooling stage 20 and, correspondingly, to the current lead 14, heat from the second cooling stage 20 will partially reach the current lead 14 along the second HTS portion of the current lead. Since the HTS material of the second HTS portion 14b of the current lead has a relatively low thermal conductivity, little heat reaching the second cooling stage 20 will be transferred to the current lead 14. Thus, little heat reaching the second cooling stage 20 will be transferred to the magnet coils 10 or the first LTS stage 14a of the current lead 14. The heat transfer to the magnet coils 10 or the first LTS stage 14a of the current lead 14 is reduced compared to the conventional arrangement of fig. 1. An advantage of this embodiment is that, in operation, the heat inflow through first cooling stage 18 to second HTS portion 14b is extracted from current lead 14 at second cooling stage 20. Thus, it is not necessary for the first LTS-portion 14a to have a high thermal conductivity as is the case in the embodiment of fig. 2. The wires of the first LTS-portion 14a can thus be kept relatively thin and easy to handle. On the other hand, the additional consumption of HTS material will increase the material cost compared to the embodiment of fig. 2.

An example advantage of the present invention is that it allows the magnet to remain in the magnetic field during a cooling fault for a long period of time, referred to as a "ride-through" period, as compared to conventional arrangements that do not benefit from the present invention. In certain embodiments, and preferably, this advantage allows the magnet to remain superconducting and in the magnetic field until cooling is restored.

Another example advantage of the present invention is that the first LTS portion 14a remains in a superconducting state during this lengthy period of time, and thus the magnet can be ramped down by orderly removing current from the magnet coils near the end of the ride-through period, thereby avoiding a quench. In a corresponding preferred embodiment, the superconducting magnet may remain superconducting while ramping down, even when cryocooler 16 is not operating. Because the first LTS portion 14a of the current lead 14 is not thermally attached to the cold head second cooling stage 20, the upper end of the first LTS portion 14a of the current lead 14 remains at a temperature below the superconducting transition temperature, which may be 8K, for example, for a long period of time after the refrigerator 16 stops operating. This allows the magnet to be ramped down by orderly removal of current from the magnet coils in a controlled manner, thereby extracting stored energy and avoiding a quench, which means that the magnet will be re-cooled and ready to be ramped back to the magnetic field more quickly after cooling is restored than after a quench.

Example materials for the first LTS portion 14a of the current lead 14 include niobium titanium or niobium tin Nb with a base material of copper or aluminum₃LTS superconductors of Sn. Suitable dimensions include any suitable length/cross-sectional area ratio. In particular embodiments, the LTS fraction may be about 0.7m long and have a length of 45mm²Cross-sectional area of (a).

Example materials for second HTS portion 14b of current lead 14 include 1G or 2G HTS tape such as BSCCO, rare earth BCO (YBCO, GdBCO) that may be used, preferably without a copper matrix material.

Example materials for the third non-superconducting 14c of current lead 14 include brass or copper or a combination thereof. Stainless steel may also be used, but a larger cross-sectional area may be required due to the resistivity of stainless steel. According to the invention, the low thermal resistance thermal link between the second cooling stage 20 and the first LTS portion of the current lead 14 and the magnet coil 10 is removed and replaced by a high thermal resistance link. The high thermal resistance may be provided by the length of second HTS portion 14b of current lead 14, or may be provided by thermal or thermal and mechanical separation of current lead 14 from second cooling stage 20 as provided in the embodiments of fig. 3 and 2, respectively.

In another embodiment of the present invention, as shown in fig. 4, a heat drain 26 is provided, the heat drain 26 being in thermal contact with the first LTS portion 14a of the current lead 14 and with the thermal link 22. The heat drain 26 is electrically insulated from one or both of the first LTS portion 14a of the current lead 14 and the heat link 22. The heat drain 26 intercepts heat flowing down the LTS portion 14a during normal operation so that the heat flows directly into the heat link 22 and back to the refrigerator second stage 20, rather than flowing into the heat link 22 via the magnet 10 and back to the refrigerator second stage 20. This increases the thermal margin of the coil 10, since the temperature of the coil 10 will be lower if the coil 10 does not transfer heat. In the example, first, the LTS portion 14a of the current lead 14 is welded to the heat drain 26 in the form of a copper block. The copper block heat drain 26 is bonded to the thermal link 22 over a large area using a thin layer of electrically insulating adhesive.

Claims

1. A current lead arrangement for providing current to a superconducting magnet coil (10), the current lead arrangement comprising a current lead (14) and a cryocooler (16),

the current lead (14) comprising a first portion of Low Temperature Superconductor (LTS) wire (14a), the first portion of low temperature superconductor wire (14a) being bonded to a second portion of High Temperature Superconductor (HTS) material (14b), and further bonded to a third portion of resistive material (14c),

the first portion (14a), the second portion (14b) and the third portion (14c) extending in this order, generally upwardly away from the superconducting magnet coils,

the cryocooler (16) comprising a first cooling stage (18) and a second cooling stage (20), the first stage being located above the second stage,

wherein a lower end of the third portion (14c) and an upper end of the second portion (14b) are thermally coupled to the first cooling stage (18), a lower end of the first portion (14a) is thermally and electrically connected to the superconducting magnet coil (10), and

wherein a thermal coupling (22) is also provided in thermal contact with the second cooling stage (20) and the superconducting magnet coil (10), the thermal coupling being provided with a thermal switch (24) to interrupt the thermal conductivity of the thermal coupling,

characterized in that the lower end of the second portion (14b) and the upper end of the first portion (14a) are thermally insulated from the second cooling stage (20).

2. The arrangement of claim 1, wherein a lower end of the second portion (14b) and an upper end of the first portion (14a) are mechanically separated from the second cooling stage (20).

3. The arrangement according to claim 1, wherein a lower end of the second portion (14b) and an upper end of the first portion (14a) are mechanically joined to the second cooling stage (20) by a thermally insulating material (26).

4. The arrangement of claim 1, wherein the second portion (14b) is partially thermally coupled to the second cooling stage (20) along the second portion (14 b).

5. The arrangement of any preceding claim, wherein a heat drain (26) is provided in thermal contact with the second cooling stage (20) and the first portion (14a), the heat drain (26) being electrically insulated from one or both of the first portion (14a) and the thermal link (22).